Polycrystalline diamond construction and method of making

ABSTRACT

A superhard polycrystalline construction comprises a body of polycrystalline superhard material, comprising a mass of superhard grains exhibiting inter-granular bonding and defining a plurality of interstitial regions therebetween, the superhard grains having an associated mean free path and a non-superhard phase at least partially filling a plurality of the interstitial regions and having an associated mean free path. The median of the mean free path associated with the non-superhard phase divided by (Q3−Q1) for the non-superhard phase being greater than or equal to 0.50, where Q1 is the first quartile and Q3 is the third quartile; and the median of the mean free path associated with the superhard grains divided by (Q3−Q1) for the superhard grains being less than 0.60. The body of polycrystalline superhard material has a first surface having a surface topology comprising one or more indentations therein and/or projections therefrom. There is also disclosed a method of forming such a construction.

FIELD

This disclosure relates to superhard constructions and methods of makingsuch constructions, particularly but not exclusively to constructionscomprising polycrystalline diamond (PCD) structures attached to asubstrate, and tools comprising the same, particularly but notexclusively for use in rock degradation or drilling, or for boring intothe earth in the oil and gas industry.

BACKGROUND

Polycrystalline diamond (PCD) is an example of a superhard material(also called a superabrasive material) comprising a mass ofsubstantially inter-grown diamond grains, forming a skeletal massdefining interstices between the diamond grains. PCD material typicallycomprises at least about 80 volume % of diamond and is conventionallymade by subjecting an aggregated mass of diamond grains to an ultra-highpressure of greater than about 5 GPa, and temperature of at least about1,200° C., for example. A material wholly or partly filling theinterstices may be referred to as filler or binder material.

PCD is typically formed in the presence of a sintering aid such ascobalt, which promotes the inter-growth of diamond grains. Suitablesintering aids for PCD are also commonly referred to as asolvent-catalyst material for diamond, owing to their function ofdissolving, to some extent, the diamond and catalysing itsre-precipitation. A solvent-catalyst for diamond is understood be amaterial that is capable of promoting the growth of diamond or thedirect diamond-to-diamond inter-growth between diamond grains at apressure and temperature condition at which diamond is thermodynamicallystable. Consequently the interstices within the sintered PCD product maybe wholly or partially filled with residual solvent-catalyst material.Most typically, PCD is often formed on a cobalt-cemented tungstencarbide substrate, which provides a source of cobalt solvent-catalystfor the PCD. Materials that do not promote substantial coherentintergrowth between the diamond grains may themselves form strong bondswith diamond grains, but are not suitable solvent-catalysts for PCDsintering.

Cemented tungsten carbide which may be used to form a suitable substrateis formed from carbide particles being dispersed in a cobalt matrix bymixing tungsten carbide particles/grains and cobalt together thenheating to solidify. To form the cutting element with an ultra hardmaterial layer such as PCD or PCBN, diamond particles or grains or CBNgrains are placed adjacent the cemented tungsten carbide body in arefractory metal enclosure such as a niobium enclosure and are subjectedto high pressure and high temperature so that inter-grain bondingbetween the diamond grains or CBN grains occurs, forming apolycrystalline ultra hard diamond or polycrystalline CBN layer.

In some instances, the substrate may be fully cured prior to attachmentto the ultra hard material layer whereas in other cases, the substratemay be green, that is, not fully cured. In the latter case, thesubstrate may fully cure during the HTHP sintering process. Thesubstrate may be in powder form and may solidify during the sinteringprocess used to sinter the ultra hard material layer.

PCD material may be used as an abrasive compact in a wide variety oftools for cutting, machining, milling, grinding, drilling or degradinghard or abrasive materials such as rock, metal, ceramics, composites andwood-containing materials. For example, tool inserts comprising PCDmaterial are widely used within drill bits used for boring into theearth in the oil and gas drilling industry. The working life ofsuperhard tool inserts may be limited by fracture of the superhardmaterial, including by spalling and chipping, or by wear of the toolinsert.

In many of these applications, the temperature of the PCD material maybecome elevated as it engages rock or other workpieces or bodies.Mechanical properties of PCD material such as abrasion resistance,hardness and strength tend to deteriorate at elevated temperatures,which may be promoted by the residual catalyst material within the bodyof PCD material.

Ever increasing drives for improved productivity in the earth boringfield place ever increasing demands on the materials used for cuttingrock. Specifically, PCD materials with improved abrasion and impactresistance are required to achieve faster cut rates and longer toollife.

Cutting elements or tool inserts comprising PCD material are widely usedin drill bits for boring into the earth in the oil and gas drillingindustry where rock drilling and other operations require high abrasionresistance and impact resistance. One of the factors limiting thesuccess of the polycrystalline diamond (PCD) abrasive cutters is thegeneration of heat due to friction between the PCD and the workmaterial. This heat causes the thermal degradation of the diamond layer.The thermal degradation increases the wear rate of the cutter throughincreased cracking and spalling of the PCD layer as well as backconversion of the diamond to graphite causing increased abrasive wear.

It is desirable to improve the abrasion resistance of a body of PCDmaterial when used as an abrasive compact in tools such as thosementioned above, as this allows extended use of the cutter, drill ormachine in which the abrasive compact is located. This is typicallyachieved by manipulating variables such as average diamondparticle/grain size, overall binder content, particle density and thelike. Methods used to improve the abrasion resistance of a PCD compositeoften result in a decrease in impact resistance of the composite.

For example, it is well known in the art to increase the abrasionresistance of an ultrahard composite by reducing the overall grain sizeof the component ultrahard particles. Typically, however, as thesematerials are made more wear resistant they become more brittle or proneto fracture.

Abrasive compacts designed for improved wear performance will thereforetend to have poor impact strength or reduced resistance to spalling.This trade-off between the properties of impact resistance and wearresistance makes designing optimised abrasive compact structures,particularly for demanding applications, inherently self-limiting.

Additionally, because finer grained structures will typically containmore solvent/catalyst or metal binder, they tend to exhibit reducedthermal stability when compared to coarser grained structures. Thisreduction in optimal behaviour for finer grained structures can causesubstantial problems in practical applications where the increased wearresistance is nonetheless required for optimal performance.

Prior art methods to solve this problem have typically involvedattempting to achieve a compromise by combining the properties of bothfiner and coarser ultrahard particle grades in various manners withinthe ultrahard abrasive layer.

Another conventional solution is to remove, typically by acid leaching,the catalyst/solvent or binder phase from the PCD material.

It is typically extremely difficult and time consuming to remove thebulk of a metallic catalyst/solvent effectively from a PCD table,particularly from the thicker PCD tables required by currentapplications. Achieving appreciable leaching depths can take so long asto be commercially unfeasible or require undesirable interventions suchas extreme acid treatment or physical drilling of the PCD tables.

It has further been appreciated that cutters and machine tool cuttinginserts having cutting surfaces with shaped topologies may beadvantageous in various applications as the surface features may bebeneficial in use to divert, for example, chips from the working surfacebeing worked on by the cutter or machine tool, and/or in some instancesto act as a chip breaker. Also, such surface topologies may producedemonstrably better surface finish qualities compared to flat surfacecutting tool geometries. However, the extreme hardness and abrasionresistance of materials such as PCD or PCBN which are typically used asthe cutting element or insert in such applications makes it verydifficult and expensive to machine these materials with desired surfacetopology features that may be used, for example, as chip breakers or todivert the debris generated in use.

There is a need to provide super-hard bodies of polycrystalline materialsuch as inserts for cutting or machine tools having effectiveperformance and to provide a more efficient method for making bodies ofpolycrystalline materials for use as such cutters or inserts. Anabrasive compact that can also achieve improved properties of abrasionresistance, fracture and impact resistance and a method of forming suchcomposites are highly desirable.

SUMMARY

Viewed from a first aspect there is provided a superhard polycrystallineconstruction comprising a body of polycrystalline superhard material,comprising:

a mass of superhard grains exhibiting inter-granular bonding anddefining a plurality of interstitial regions therebetween, the superhardgrains having an associated mean free path; anda non-superhard phase at least partially filling a plurality of theinterstitial regions and having an associated mean free path;the median of the mean free path associated with the non-superhard phasedivided by (Q3−Q1) for the non-superhard phase being greater than orequal to 0.50, where Q1 is the first quartile and Q3 is the thirdquartile; andthe median of the mean free path associated with the superhard grainsdivided by (Q3−Q1) for the superhard grains being less than 0.60;wherein the body of polycrystalline superhard material has a firstsurface having a surface topology comprising one or more indentationstherein and/or projections therefrom.

Viewed from a second aspect there is provided a method of forming asuperhard polycrystalline construction, comprising:

-   -   providing a mass of grains of superhard material; and    -   treating the pre-sinter assembly in the presence of a        catalyst/solvent material for the superhard grains at an        ultra-high pressure of around 5.5 GPa or greater and a        temperature at which the superhard material is more        thermodynamically stable than graphite to sinter together the        grains of superhard material to form a polycrystalline superhard        construction, the superhard grains exhibiting inter-granular        bonding and defining a plurality of interstitial regions        therebetween, a non-superhard phase at least partially filling a        plurality of the interstitial regions;    -   wherein:        -   the median of the mean free path associated with the            non-superhard phase divided by (Q3−Q1) for the non-superhard            phase is greater than or equal to 0.50, where Q1 is the            first quartile and Q3 is the third quartile of the mean free            path measurements associated with the non-superhard phase;            and        -   the median of the mean free path associated with the            superhard grains divided by (Q3−Q1) for the superhard grains            is less than 0.60, where Q1 is the first quartile and Q3 is            the third quartile of the mean free path measurements            associated with the superhard grains; and        -   the method further comprising forming a non-planar surface            topology in a first surface of the body of polycrystalline            diamond material, the surface topology comprising one or            more indentations in and/or projections extending from the            first surface.

BRIEF DESCRIPTION OF DRAWINGS

Non-limiting embodiments will now be described by way of example andwith reference to the accompanying drawings in which:

FIG. 1 is a schematic drawing of the microstructure of a body of PCDmaterial; and

FIG. 2 is a schematic drawing of a PCD compact comprising a PCDstructure bonded to a substrate.

DETAILED DESCRIPTION

As used herein, “polycrystalline diamond” (PCD) material comprises amass of diamond grains, a substantial portion of which are directlyinter-bonded with each other and in which the content of diamond is atleast about 80 volume percent of the material. In one embodiment of PCDmaterial, interstices between the diamond gains may be at least partlyfilled with a binder material comprising a catalyst for diamond. As usedherein, “interstices” or “interstitial regions” are regions between thediamond grains of PCD material. In embodiments of PCD material,interstices or interstitial regions may be substantially or partiallyfilled with a material other than diamond, or they may be substantiallyempty. Embodiments of PCD material may comprise at least a region fromwhich catalyst material has been removed from the interstices, leavinginterstitial voids between the diamond grains.

As used herein, a “PCD structure” comprises a body of PCD material.

As used herein, a “metallic” material is understood to comprise a metalin unalloyed or alloyed form and which has characteristic properties ofa metal, such as high electrical conductivity.

As used herein, “catalyst material” for diamond, which may also bereferred to as solvent/catalyst material for diamond, means a materialthat is capable of promoting the growth of diamond or the directdiamond-to-diamond inter-growth between diamond grains at a pressure andtemperature condition at which diamond is thermodynamically stable.

A filler or binder material is understood to mean a material that whollyor partially fills pores, interstices or interstitial regions within apolycrystalline structure.

A multi-modal size distribution of a mass of grains is understood tomean that the grains have a size distribution with more than one peak,each peak corresponding to a respective “mode”. Multimodalpolycrystalline bodies may be made by providing more than one source ofa plurality of grains, each source comprising grains having asubstantially different average size, and blending together the grainsor particles from the sources. In one embodiment, the PCD structure maycomprise diamond grains having a multimodal distribution.

As used herein, the term ‘total binder area’ is expressed as thepercentage of non-diamond phase(s) in the total cross-sectional area ofa polished cross section of the body of PCD material being analysed.

As used herein, a “superhard material” is a material having a Vickershardness of at least about 28 GPa. Diamond and cubic boron nitride (cBN)material are examples of superhard materials.

As used herein, a “superhard construction” means a constructioncomprising a body of polycrystalline superhard material. In such aconstruction, a substrate may be attached thereto or alternatively thebody of polycrystalline material may be free-standing and unbacked.

As used herein, PCBN (polycrystalline cubic boron nitride) materialrefers to a type of superhard material comprising grains of cubic boronnitride (cBN) dispersed within a matrix comprising metal or ceramic.PCBN is an example of a superhard material.

The term “substrate” as used herein means any substrate over which theultra hard material layer is formed. For example, a “substrate” as usedherein may be a transition layer formed over another substrate.Additionally, as used herein, the terms “radial” and “circumferential”and like terms are not meant to limit the feature being described to aperfect circle.

As used herein, the term “integrally formed” regions or parts areproduced contiguous with each other and are not separated by a differentkind of material.

Like reference numbers are used to identify like features in alldrawings.

With reference to FIG. 1, a body of PCD material 10 comprises a mass ofdirectly inter-bonded grains of superhard material 12 and interstices 14between the grains 12, which may be at least partly filled with filleror binder material. FIG. 2 shows an embodiment of a superhard compositecompact 20 for use as a cutter comprising a body of superhard material22 integrally bonded at an interface 24 to a substrate 30. The substrate30 may be formed of, for example, cemented carbide material and may be,for example, cemented tungsten carbide, cemented tantalum carbide,cemented titanium carbide, cemented molybdenum carbide or mixturesthereof. The binder metal for such carbides may be, for example, nickel,cobalt, chromium, iron or an alloy containing one or more of thesemetals. Typically, this binder will be present in an amount of 10 to 20mass %, but this may be as low as 6 mass % or less. Some of the bindermetal may infiltrate the body of polycrystalline superhard material 22during formation of the compact 20.

The compact 20 of FIG. 2 may, in use, be attached to a drill bit (notshown) for oil and gas drilling operations. The body of superhardmaterial 10 has a free exposed surface 36, which is the surface which,along with its edge, performs the cutting in use. This surface has anon-planar surface topology 38 with surface features extending fromand/or into the free surface. In embodiments where the compact 20 is tobe used as a cutter, for example for drilling in the oil and gasindustry, the surface topology may be used to direct or divert the rockor earth away from the drill bit to which the cutter is attached.Alternatively or additionally, the surface topology may act as a chipbreaker suitable for controlling aspects of the size and shape of chipsformed when the body of polycrystalline superhard material is used, forexample, as a cutter or as an insert for a machine tool to machine aworkpiece. Such topology may include depression and/or protrusionfeatures, such as radial or peripheral ridges and troughs, formed on arake surface of the insert.

An example of a method for producing the PCD compact 20 comprising thebody of PCD material 22, as shown in FIGS. 1 and 2, is now described.

In some embodiments, the body of superhard material 22 may include, forexample, one or more of nanodiamond additions in the form of nanodiamondpowder up to 20 wt %, salt systems, borides, metal carbides of Ti, V, Nbor any of the metals Pd or Ni.

The grains of superhard material may be for example diamond grains orparticles. In the starting mixture prior to sintering they may be, forexample, bimodal, that is, the feed comprises a mixture of a coarsefraction of diamond grains and a fine fraction of diamond grains. Insome embodiments, the coarse fraction may have, for example, an averageparticle/grain size ranging from about 10 to 60 microns. By “averageparticle or grain size” it is meant that the individual particles/grainshave a range of sizes with the mean particle/grain size representing the“average”. The average particle/grain size of the fine fraction is lessthan the size of the coarse fraction, for example between around 1/10 to6/10 of the size of the coarse fraction, and may, in some embodiments,range for example between about 0.1 to 20 microns.

In some embodiments, the weight ratio of the coarse diamond fraction tothe fine diamond fraction ranges from about 50% to about 97% coarsediamond and the weight ratio of the fine diamond fraction may be fromabout 3% to about 50%. In other embodiments, the weight ratio of thecoarse fraction to the fine fraction will range from about 70:30 toabout 90:10.

In further embodiments, the weight ratio of the coarse fraction to thefine fraction may range for example from about 60:40 to about 80:20.

In some embodiments, the particle size distributions of the coarse andfine fractions do not overlap and in some embodiments the different sizecomponents of the compact are separated by an order of magnitude betweenthe separate size fractions making up the multimodal distribution.

The embodiments consist of at least a wide bi-modal size distributionbetween the coarse and fine fractions of superhard material, but someembodiments may include three or even four or more size modes which may,for example, be separated in size by an order of magnitude, for example,a blend of particle sizes whose average particle size is 20 microns, 2microns, 200 nm and 20 nm.

Sizing of diamond particles/grains into fine fraction, coarse fraction,or other sizes in between, may be through known processes such asjet-milling of larger diamond grains and the like.

In embodiments where the superhard material is polycrystalline diamondmaterial, the diamond grains used to form the polycrystalline diamondmaterial may be natural or synthetic.

In some embodiments, the binder catalyst/solvent may comprise cobalt orsome other iron group elements, such as iron or nickel, or an alloythereof. Carbides, nitrides, borides, and oxides of the metals of GroupsIV-VI in the periodic table are other examples of non-diamond materialthat might be added to the sinter mix. In some embodiments, thebinder/catalyst/sintering aid may be Co.

The cemented metal carbide substrate may be conventional in compositionand, thus, may be include any of the Group IVB, VB, or VIB metals, whichare pressed and sintered in the presence of a binder of cobalt, nickelor iron, or alloys thereof. In some embodiments, the metal carbide istungsten carbide.

In some embodiments, both the bodies of, for example, diamond andcarbide material plus sintering aid/binder/catalyst are applied aspowders and are sintered simultaneously in a single UHP/HT process. Thediamond grains and mass of carbide to form the substrate are placed inan HP/HT reaction cell assembly and subjected to HP/HT processing. TheHP/HT processing conditions selected are sufficient to effectintercrystalline bonding between adjacent grains of abrasive particlesand, optionally, the joining of sintered particles to the cemented metalcarbide support. In one embodiment, the processing conditions generallyinvolve the imposition for about 3 to 120 minutes of a temperature of atleast about 1200 degrees C. and an ultra-high pressure of greater thanabout 5 GPa.

In some embodiments, the substrate may be pre-sintered in a separateprocess before being bonded together in the HP/HT press during sinteringof the ultrahard polycrystalline material.

In a further embodiment, both the substrate and a body ofpolycrystalline superhard material are pre-formed. For example, thebimodal or multimodal feed of ultrahard grains/particles with optionalcarbonate binder-catalyst also in powdered form are mixed together, andthe mixture is packed into an appropriately shaped canister and is thensubjected to extremely high pressure and temperature in a press.Typically, the pressure is at least 5 GPa and the temperature is atleast around 1200 degrees C. The preformed body of polycrystallinesuperhard material is then placed in the appropriate position on theupper surface of the preform carbide substrate (incorporating a bindercatalyst), and the assembly is located in a suitably shaped canister.The assembly is then subjected to high temperature and pressure in apress, the order of temperature and pressure being again, at leastaround 1200 degrees C. and 5 GPa respectively. During this process thesolvent/catalyst migrates from the substrate into the body of superhardmaterial and acts as a binder-catalyst to effect intergrowth in thelayer and also serves to bond the layer of polycrystalline superhardmaterial to the substrate. The sintering process also serves to bond thebody of superhard polycrystalline material to the substrate.

A support body comprising cemented carbide in which the cement or bindermaterial comprises a catalyst material for diamond, such as cobalt, maybe provided. The support body may have a non-planar end or asubstantially planar proximate end on which the PCD structure is to beformed and which forms the interface 24. A non-planar shape of the endmay be configured to reduce undesirable residual stress between the PCDstructure 22 and the support body 30. A cup may be provided for use inassembling the diamond-containing sheets onto the support body. Thefirst and second sets of discs may be stacked into the bottom of thecup. In one version of the method, a layer of substantially loosediamond grains may be packed onto the uppermost of the discs. Thesupport body may then be inserted into the cup with the proximate endgoing in first and pushed against the substantially loose diamondgrains, causing them to move slightly and position themselves accordingto the shape of the non-planar end of the support body to form apre-sinter assembly.

The pre-sinter assembly may be placed into a capsule for an ultra-highpressure press and subjected to an ultra-high pressure of at least about5.5 GPa and a high temperature of at least about 1,300 degreescentigrade to sinter the diamond grains and form a PCD elementcomprising a PCD structure integrally joined to the support body. In oneversion of the method, when the pre-sinter assembly is treated at theultra-high pressure and high temperature, the binder material within thesupport body melts and infiltrates the strata of diamond grains. Thepresence of the molten catalyst material from the support body is likelyto promote the sintering of the diamond grains by intergrowth with eachother to form an integral, stratified PCD structure.

In some versions of the method, the aggregate masses may comprisesubstantially loose diamond grains, or diamond grains held together by abinder material. The aggregate masses of multimodal grains may be in theform of granules, discs, wafers or sheets, and may contain catalystmaterial for diamond and/or additives for reducing abnormal diamondgrain growth, for example, or the aggregated mass may be substantiallyfree of catalyst material or additives. In some embodiments, theaggregate masses may be assembled onto a cemented carbide support body.

In some embodiments, the pre-sinter assembly may be subjected to apressure of at least about 6 GPa, at least about 6.5 GPa, at least about7 GPa or even at least about 7.5 GPa.

The one or more indentations in and/or projections 38 from the freecutting surface 36 of the body of PCD material 22 may be formed duringthe sintering process or may, for example, be formed post-sinteringusing techniques such as electrical discharge machining (EDM) or laserablation to achieve the desired surface topology to suit the applicationin which the compact is to be employed.

An example method of forming the shaped surface topology during thesintering process is set out below.

The aggregated mass of grains of diamond material is placed into acanister, and a ceramic punch or layer formed of a ceramic materialwhich does not react chemically with the diamond material is placed incontact with the aggregated mass of grains of diamond material, theceramic layer having a surface with surface topology. The ceramicmaterial may additionally or alternatively be such that it does notreact chemically with the sinter catalyst material used to bond thediamond grains to one another during sintering. In some embodiments, thesurface topology of the ceramic material is placed in direct contactwith the diamond grains to imprint a pattern therein complementary tothe surface topology. In other embodiments, the ceramic material may bein indirect contact with the grains, being spaced therefrom by a thinlayer or a coating to assist in post sintering separation of the ceramicmaterial from the sintered superhard diamond material. In such cases,any coating or additional layer is also formed of a material that doesnot react chemically with the superhard material and/or the sintercatalyst material. The aggregated mass of diamond grains and ceramiclayer are then subjected to an ultra-high pressure of at least about 5.5GPa and a temperature of at least about 1,250 degrees centigrade to meltthe cobalt comprised in the substrate body and sinter the diamond grainsto each other to form a body of polycrystalline superhard materialhaving a surface topology complementary to the surface topology of theceramic layer. The ceramic layer is then removed from the body ofpolycrystalline material for example by impact.

The ceramic layer may be easily removed from the body of polycrystallinematerial as there is no chemical reaction with the ceramic materialenabling easy separation of the two bodies. Any residual ceramic may beremoved by a light sand blast, resulting in a good, semi-polishedsurface finish. The ceramic materials that may be used to create thesurface topology in the superhard material may include, for example, thegroup of oxide ceramic materials that are not reduced by carbo-thermalreaction, including Magnesia, Calcia, Zirconia, Alumina.

As mentioned above, in some embodiments, the surface topology of theceramic material may be coated with a layer which directly contacts thegrains prior to sintering and which is of a composition such that itfacilitates removal of the ceramic body from the sintered body ofpolycrystalline superhard material. Examples of such a coating mayinclude zirconia, alumina, calcium carbonate or calcium oxide.

In alternative embodiments, the ceramic material directly contacts thegrains of polycrystalline superhard material to be sintered.

The step of placing the grains of superhard material into the canistermay, in some embodiments, comprise providing a plurality of sheetscomprising the grains and stacking the sheets in the canister to formthe aggregation of superhard grains. In other embodiments, the grains ofsuperhard material may be deposited into the canister usingsedimentation or electrophoretic deposition techniques.

In some embodiments, the ceramic material may be formed, for example, ofany one or more of the group of oxide ceramic materials that are notreduced by carbo-thermal reaction in contact with the grains. An exampleof such materials may include any one or more of the group of oxideceramic materials comprising oxides of magnesia, calcia, zirconia,and/or alumina.

The steps of placing the materials in the canister may be reversed ortheir order changed, for example, the step of placing the ceramic layerin contact with the aggregated mass of grains may be after the step ofplacing the grains into a canister. Alternatively, the ceramic layer maybe placed into the canister before the grains are placed in thecanister.

The body of polycrystalline diamond material formed by this method mayhave a free outer surface 36, on removal of the ceramic layer therefrom,which is of the same quality as the bulk of the body of polycrystallinematerial. This is in contrast, for example, to conventionally formed PCDin which the PCD layer in direct contact with the canister material usedduring sintering is usually of an inferior quality compared to the bulkPCD due to an interaction between the diamond, cobalt binder andcanister material. Thus, in conventional PCD cutters, it is usuallynecessary to remove the top surface by grinding, sandblasting or othermethods. Such steps are not required in PCD formed according to one ormore embodiments as the body of polycrystalline superhard material has asurface topology on a first surface, the first surface beingsubstantially free of material from a canister used in formation of thebody of polycrystalline superhard material.

The surface topology of the ceramic material may be designed accordingto the requirements of a given application of the polycrystalline bodyand having regard to the intended shape of the body depending on itsultimate use. For example, in some embodiments the surface topology ofthe ceramic material is constructed to impart a chamfered edge to thebody of polycrystalline superhard material during sintering.

In some embodiments, such as those illustrated in FIG. 2, the body ofPCD material 22 may be formed on a substrate 30, the substrate beingplaced into the canister prior to sintering, the body of polycrystallinesuperhard material 22 bonding to the substrate 30 during sintering alongan interface therebetween. The interface 24 may be substantially planar,such as shown in FIG. 2, or it may be substantially non-planar.

The substrate 30 may, for example, be formed of cemented carbidematerial. In some embodiments, the sintered body may have a thickness ofup to around 6000 microns.

After forming the body of sintered polycrystalline material, a finishingtreatment may be applied to treat the body of super-hard material 22 toremove sinter catalyst from at least some of the interstices between theinter-bonded grains. In particular, catalyst material may be removedfrom a region of the PCD structure 22 adjacent the working surface orthe side surface or both the working surface and the side surface. Thismay be done by treating the PCD structure 22 with acid to leach outcatalyst material from between the diamond grains, or by other methodssuch as electrochemical methods. A thermally stable region, which may besubstantially porous, extending a depth of at least about 50 microns orat least about 100 microns from a surface of the PCD structure 22, maythus be provided. Some embodiments with 50 to 80 micron thick layers inwhich this leach depth is around 250 microns have been shown to exhibitsubstantially improved performance, for example a doubling inperformance after leaching over an unleached PCD product. In oneexample, the substantially porous region may comprise at most 2 weightpercent of catalyst material.

In embodiments where the cemented carbide substrate does not containsufficient solvent/catalyst for diamond, and where the PCD structure isintegrally formed onto the substrate during sintering at an ultra-highpressure, solvent/catalyst material may be included or introduced intothe aggregated mass of diamond grains from a source of the materialother than the cemented carbide substrate. The solvent/catalyst materialmay comprise cobalt that infiltrates from the substrate in to theaggregated mass of diamond grains just prior to and during the sinteringstep at an ultra-high pressure. However, in embodiments where thecontent of cobalt or other solvent/catalyst material in the substrate islow, particularly when it is less than about 11 weight percent of thecemented carbide material, then an alternative source may need to beprovided in order to ensure good sintering of the aggregated mass toform PCD.

Solvent/catalyst for diamond may be introduced into the aggregated massof diamond grains by various methods, including blendingsolvent/catalyst material in powder form with the diamond grains,depositing solvent/catalyst material onto surfaces of the diamondgrains, or infiltrating solvent/catalyst material into the aggregatedmass from a source of the material other than the substrate, eitherprior to the sintering step or as part of the sintering step. Methods ofdepositing solvent/catalyst for diamond, such as cobalt, onto surfacesof diamond grains are well known in the art, and include chemical vapourdeposition (CVD), physical vapour deposition (PVD), sputter coating,electrochemical methods, electroless coating methods and atomic layerdeposition (ALD). It will be appreciated that the advantages anddisadvantages of each depend on the nature of the sintering aid materialand coating structure to be deposited, and on characteristics of thegrain.

In one embodiment, cobalt may be deposited onto surfaces of the diamondgrains by first depositing a pre-cursor material and then converting theprecursor material to a material that comprises elemental metalliccobalt. For example, in the first step cobalt carbonate may be depositedon the diamond grain surfaces using the following reaction:

Co(NO₃)₂+Na₂CO₃->CoCO₃+2NaNO₃

The deposition of the carbonate or other precursor for cobalt or othersolvent/catalyst for diamond may be achieved by means of a methoddescribed in PCT patent publication number WO/2006/032982. The cobaltcarbonate may then be converted into cobalt and water, for example, bymeans of pyrolysis reactions such as the following:

CoCO₃->CoO+CO₂

CoO+H₂->CO+H₂O

In another embodiment, cobalt powder or precursor to cobalt, such ascobalt carbonate, may be blended with the diamond grains. Where aprecursor to a solvent/catalyst such as cobalt is used, it may benecessary to heat treat the material in order to effect a reaction toproduce the solvent/catalyst material in elemental form before sinteringthe aggregated mass.

As described above, to assist in improving thermal stability of thesintered structure, the catalysing material may be removed from a regionof the polycrystalline layer adjacent an exposed surface thereof.Generally, that surface will be on a side of the polycrystalline layeropposite to the substrate and will provide a working surface for thepolycrystalline diamond layer. Removal of the catalysing material may becarried out using methods known in the art such as electrolytic etching,and acid leaching and evaporation techniques.

It has been found that multimodal distributions of some embodiments mayassist in achieving a very high degree of diamond intergrowth whilestill maintaining sufficient open porosity to enable efficient leaching.

Polycrystalline bodies formed according to the above-described methodmay have many applications. For example, they may be used as an insertfor a machine tool, in which the cutter structure comprises the body ofpolycrystalline superhard material according to one or more embodimentsand the surface topology of the polycrystalline material in such anapplication may be used as a chip-breaker. In such inserts, the cutterstructure which may be joined to an insert base, may have, for example,a mean thickness of at least 100 microns, and in some embodiments, amean thickness of at most 1,000 microns.

Embodiments are described in more detail below with reference to thefollowing examples which are provided herein by way of illustration onlyand are not intended to be limiting.

Example 1

This non-limiting example illustrates a method of forming the surfacetopology during the sintering process.

A surface topology configuration may be designed according to therequirements of a given drilling or machining application and havingregard to the intended shape of a cutter structure or machine toolinsert. A cobalt-cemented carbide substrate body may be provided and aceramic plug may be provided, the ceramic plug having a surfacecomprising a surface topology that is complementary (i.e. inverse) tothat of the desired surface topology for the cutter or machine toolinsert. A pre-compact assembly may be prepared by forming a plurality ofdiamond grains into an aggregation against the surface of the substrate,and encapsulating the assembly within a jacket, formed for example ofalumina or other ceramic material. The surface of the ceramic plughaving the desired surface topology to be imparted to the diamond bodyon sintering is placed in contact with the diamond grains. Thepre-compact assembly is subjected to an ultra-high pressure of at leastabout 5.5 GPa and a temperature of at least about 1,250 degreescentigrade to melt the cobalt comprised in the substrate body and sinterthe diamond grains to each other to form a composite compact comprisinga PCD structure formed joined to the substrate. After sintering, theceramic plug may be removed from the sintered PCD material by, forexample, light impact and the PCD structure may be treated in acid toremove residual cobalt within interstitial regions between theinter-grown diamond grains. Removal of a substantial amount of cobaltfrom the PCD structure is likely to increase substantially the thermalstability of the PCD structure and will likely reduce the risk ofdegradation of the PCD material. The composite compact thus formed maybe further processed, depending on its intended application. Forexample, if it is to be used as a machine tool insert, it may be furthertreated by grinding to provide a machine tool insert comprising the PCDcutter structure having well-defined chip-breaker features.

Example 2

A quantity of sub-micron cobalt powder sufficient to obtain 2 mass % inthe final diamond mixture was initially de-agglomerated in a methanolslurry in a ball mill with WC milling media for 1 hour. A fine fractionof diamond powder with an average grain size of 2 □m was then added tothe slurry in an amount to obtain 10 mass % in the final mixture.Additional milling media was introduced and further methanol was addedto obtain suitable slurry; and this was milled for a further hour. Acoarse fraction of diamond, with an average grain size of approximately20 □m was then added in an amount to obtain 88 mass % in the finalmixture. The slurry was again supplemented with further methanol andmilling media, and then milled for a further 2 hours. The slurry wasremoved from the ball mill and dried to obtain the diamond powdermixture.

The diamond powder mixture was then placed into a suitable HpHT vessel,adjacent to a tungsten carbide substrate and sintered at a pressure ofaround 6.8 GPa and a temperature of about 1500 □C.

The surface topology 38 in the cutting surface 36 of the PCD body 22 wasformed post sintering using EDM techniques. In other embodiments, thesurface topology could have been formed during sintering using, forexample, the techniques described above in example 1.

Example 3

A quantity of sub-micron cobalt powder sufficient to obtain 2.4 mass %in the final diamond mixture was initially de-agglomerated in a methanolslurry in a ball mill with WC milling media for 1 hour. A fine fractionof diamond powder with an average grain size of 2 □m was then added tothe slurry in an amount to obtain 29.3 mass % in the final mixture.Additional milling media was introduced and further methanol was addedto obtain a suitable slurry; and this was milled for a further hour. Acoarse fraction of diamond, with an average grain size of approximately20 □m was then added in an amount to obtain 68.3 mass % in the finalmixture. The slurry was again supplemented with further methanol andmilling media, and then milled for a further 2 hours. The slurry wasremoved from the ball mill and dried to obtain the diamond powdermixture.

The diamond content of the sintered diamond structure is greater than 90vol % and the coarsest fraction of the distribution may, in someembodiments, be greater than 60 weight % or greater than weight 70%.

The surface topology 38 in the cutting surface 36 of the PCD body 22 wasformed post sintering using EDM techniques. In other embodiments, thesurface topology could have been formed during sintering using, forexample, the techniques described above in example 1.

The surface topology of the ceramic material may be designed accordingto the requirements of a given application of the polycrystalline bodyand having regard to the intended shape of the body depending on itsultimate use. For example, in some embodiments the surface topology ofthe ceramic material is constructed to impart a chamfered edge to thebody of polycrystalline superhard material during sintering.

In some embodiments, the polycrystalline bodies formed according to theabove-described methods may be used as a cutter for boring into theearth, or as a PCD element for a rotary shear bit for boring into theearth, or for a percussion drill bit or for a pick for mining or asphaltdegradation. Alternatively, a drill bit or a component of a drill bitfor boring into the earth, may comprise the body of polycrystallinesuperhard material according to any one or more embodiments.

In polycrystalline diamond material, individual diamond particles/grainsare, to a large extent, bonded to adjacent particles/grains throughdiamond bridges or necks. The individual diamond particles/grains retaintheir identity, or generally have different orientations. The averagegrain/particle size of these individual diamond grains/particles may bedetermined using image analysis techniques. Images are collected on ascanning electron microscope and are analysed using standard imageanalysis techniques. From these images, it is possible to extract arepresentative diamond particle/grain size distribution.

Generally, the body of polycrystalline diamond material will be producedand bonded to the cemented carbide substrate in a HPHT process. In sodoing, it is advantageous for the binder phase and diamond particles tobe arranged such that the binder phase is distributed homogeneously andis of a fine scale.

A cross-section through the PCD structure was then examinedmicro-structurally by means of a scanning electron microscope (SEM).

The homogeneity or uniformity of the sintered structure is defined byconducting a statistical evaluation of a large number of collectedimages. The distribution of the binder phase, which is easilydistinguishable from that of the diamond phase using electronmicroscopy, can then be measured in a method similar to that disclosedin EP 0974566. This method allows a statistical evaluation of theaverage thicknesses of the binder phase along several arbitrarily drawnlines through the microstructure. This binder thickness measurement isalso referred to as the “mean free path” by those skilled in the art.For two materials of similar overall composition or binder content andaverage diamond grain size, the material which has the smaller averagethickness will tend to be more homogenous, as this implies a “finerscale” distribution of the binder in the diamond phase. In addition, thesmaller the standard deviation of this measurement, the more homogenousis the structure. A large standard deviation implies that the binderthickness varies widely over the microstructure, i.e. that the structureis not even, but contains widely dissimilar structure types.

The binder and diamond mean free path measurements were obtained forvarious samples in the manner set out below. Unless otherwise statedherein, dimensions of mean free path within the body of PCD materialrefer to the dimensions as measured on a surface of, or a sectionthrough, a body comprising PCD material and no stereographic correctionhas been applied. For example, the measurements are made by means ofimage analysis carried out on a polished surface, and a Saltykovcorrection has not been applied in the data stated herein.

In measuring the mean value of a quantity or other statistical parametermeasured by means of image analysis, several images of different partsof a surface or section (hereinafter referred to as samples) are used toenhance the reliability and accuracy of the statistics. The number ofimages used to measure a given quantity or parameter may be, for examplebetween 10 to 30. If the analysed sample is uniform, which is the casefor PCD, depending on magnification, 10 to 20 images may be consideredto represent that sample sufficiently well.

The resolution of the images needs to be sufficiently high for theinter-grain and inter-phase boundaries to be clearly made out and, forthe measurements stated herein an image area of 1280 by 960 pixels wasused. Images used for the image analysis were obtained by means ofscanning electron micrographs (SEM) taken using a backscattered electronsignal. The back-scatter mode was chosen so as to provide high contrastbased on different atomic numbers and to reduce sensitivity to surfacedamage (as compared with the secondary electron imaging mode).

-   1. A sample piece of the PCD sintered body is cut using wire EDM and    polished. At least 10 back scatter electron images of the surface of    the sample are taken using a Scanning Electron Microscope at 1000    times magnifications.-   2. The original image was converted to a greyscale image. The image    contrast level was set by ensuring the diamond peak intensity in the    grey scale histogram image occurred between 10 and 20.-   3. An auto threshold feature was used to binarise the image and    specifically to obtain clear resolution of the diamond and binder    phases.-   4. The software, having the trade name analySIS Pro from Soft    Imaging System® GmbH (a trademark of Olympus Soft Imaging Solutions    GmbH) was used and excluded from the analysis any particles which    touched the boundaries of the image. This required appropriate    choice of the image magnification:-   a. If too low then resolution of fine particles is reduced.-   b. If too high then:-   i. Efficiency of coarse grain separation is reduced.-   ii. High numbers of coarse grains are cut by the boarders of the    image and hence less of these grains are analysed.-   iii. Thus more images must be analysed to get a    statistically-meaningful result.-   5. Each particle was finally represented by the number of continuous    pixels of which it is formed.-   6. The AnalySIS software programme proceeded to detect and analyse    each particle in the image. This was automatically repeated for    several images.-   7. Ten SEM images were analyzed using the grey-scale to identify the    binderpools as distinct from the other phases within the sample. The    threshold value for the SEM was then determined by selecting a    maximum value for binder pools content which only identifies binder    pools and excludes all other phases (whether grey or white). Once    this threshold value is identified it is used to binarize the SEM    image.)-   8. One pixel thick lines were superimposed across the width of the    binarized image, with each line being five pixels apart (to ensure    the measurement is sufficiently representative in statistical    terms). Binder phase that are cut by image boundaries were excluded    in these measurements.-   9. The distance between the binder pools along the superimposed    lines were measured and recorded—at least 10,000 measurements were    made per material being analysed. Median values were reported for    both the non-diamond phase mean free paths and diamond phase mean    free paths.-   The distance between the binder pools along the superimposed lines    were measured and recorded—at least 10,000 measurements were made    per material being analysed. The median value for the non-diamond    phase mean free paths and the diamond phase mean free paths were    calculated. The term “median” in this context is considered to have    its conventional meaning, namely the numerical value separating the    higher half of the data sample from the lower half.

Also recorded were the mean free path measurements at Q1 and Q3 for boththe diamond and non-diamond phases.

Q1 is typically referred to as the first quartile (also called the lowerquartile) and is the number below which lies the 25 percent of thebottom data. Q3 is typically referred to as the third quartile (alsocalled the upper quartile) has 75 percent of the data below it and thetop 25 percent of the data above it.

From this, it was determined that embodiments have:

alpha >=0.50 and beta <0.60,wherealpha is the non-diamond phase MFP median/(Q3−Q1), which gives a measureof “uniform binder pool size”; andbeta=diamond MFP median/(Q3−Q1) which gives a measure of “wide grainsize distribution”

In some embodiments it was determined that alpha >=0.83 and beta <0.47.

While various embodiments have been described with reference to a numberof examples, those skilled in the art will understand that variouschanges may be made and equivalents may be substituted for elementsthereof and that these examples are not intended to limit the particularembodiments disclosed. Various example arrangements and combinations forcutter structures and inserts are envisaged by the disclosure. Thecutter structure may comprise natural or synthetic diamond material.Examples of diamond material include polycrystalline diamond (PCD)material, thermally stable PCD material, crystalline diamond material,diamond material made by means of a chemical vapour deposition (CVD)method or silicon carbide bonded diamond.

Furthermore, the cutter structure described herein with reference to oneor more embodiments may be used as part of an insert for a machine tool,comprising the cutter structure with the superhard polycrystallineconstruction described herein joined to an insert base, the surfacetopology being formed on a first face of the body of polycrystallinesuperhard material, the first surface forming a rake face or a cuttingface, and the surface topology of the first surface forming chip-breakertopology.

In one or more other embodiments, the superhard polycrystallinestructure described herein may form a PCD element for one or more of arotary shear bit for boring into the earth, a percussion drill bit, or apick for mining or asphalt degradation.

1. A superhard polycrystalline construction comprising a body ofpolycrystalline superhard material, comprising: a mass of superhardgrains exhibiting inter-granular bonding and defining a plurality ofinterstitial regions therebetween, the superhard grains having anassociated mean free path; and a non-superhard phase at least partiallyfilling a plurality of the interstitial regions and having an associatedmean free path; the median of the mean free path associated with thenon-superhard phase divided by (Q3−Q1) for the non-superhard phase beinggreater than or equal to 0.50, where Q1 is the first quartile and Q3 isthe third quartile; and the median of the mean free path associated withthe superhard grains divided by (Q3−Q1) for the superhard grains beingless than 0.60; wherein the body of polycrystalline superhard materialhas a first surface having a surface topology comprising one or moreindentations therein and/or projections therefrom.
 2. A superhardpolycrystalline construction according to claim 1, wherein the superhardgrains comprise natural and/or synthetic diamond grains, the superhardpolycrystalline construction forming a polycrystalline diamondconstruction.
 3. A superhard polycrystalline construction according toclaim 1, wherein the non-superhard phase comprises a binder phase.
 4. Asuperhard polycrystalline construction according to claim 3, wherein thebinder phase comprises cobalt, and/or one or more other iron groupelements, or an alloy thereof, and/or one or more carbides, nitrides,borides, and oxides of the metals of Groups IV-VI in the periodic table.5. A superhard polycrystalline construction according to claim 4,wherein the one or more other iron group elements comprises iron ornickel. 6-7. (canceled)
 8. A superhard polycrystalline constructionaccording to claim 1, wherein the first surface comprises an externalworking surface forming the working or cutting surface of thepolycrystalline construction in use.
 9. A superhard polycrystallineconstruction according to claim 1, wherein the polycrystallineconstruction comprises one or more of: up to 20 wt % nanodiamondadditions in the form of nanodiamond powder grains; salts; borides ormetal carbides of at least one of Ti, V, or Nb; or at least one of themetals Pd or Ni.
 10. A superhard polycrystalline construction as claimedin claim 1, wherein at least a portion of the body of polycrystallinesuperhard material is substantially free of a catalyst material fordiamond, said portion forming a thermally stable region.
 11. A superhardpolycrystalline construction as claimed in claim 10, wherein thethermally stable region extends a depth of at least 50 microns from asurface of the body of polycrystalline superhard material.
 12. Asuperhard polycrystalline construction as claimed in claim 10, whereinthe thermally stable region comprising at most 2 weight percent ofcatalyst material for diamond.
 13. (canceled)
 14. A superhardpolycrystalline construction as claimed in claim 1 wherein the median ofthe mean free path associated with the non-superhard phase divided by(Q3−Q1) for the non-superhard phase being greater than or equal to 0.83.15. A superhard polycrystalline construction as claimed in claim 1wherein the median of the mean free path associated with the superhardgrains divided by (Q3−Q1) for the superhard grains is less than 0.47.16. A superhard polycrystalline construction according to claim 1,wherein the first surface is substantially free of material from acanister used in formation of the body of polycrystalline superhardmaterial.
 17. The polycrystalline superhard construction according toclaim 16, wherein the first surface is of the same quality as the bulkof the body of polycrystalline superhard material. 18-22. (canceled) 23.An insert for a machine tool, comprising a cutter structure joined to aninsert base, the cutter structure comprising the polycrystallinesuperhard construction as claimed in claim 1, the surface topology beingformed on a first face of the body of polycrystalline superhardmaterial, the first surface forming a rake face or a cutting face, andthe surface topology of the first surface forming chip-breaker topology.24-25. (canceled)
 26. A method of forming a superhard polycrystallineconstruction, comprising: providing a mass of grains of superhardmaterial; and treating the pre-sinter assembly in the presence of acatalyst/solvent material for the superhard grains at an ultra-highpressure of around 5.5 GPa or greater and a temperature at which thesuperhard material is more thermodynamically stable than graphite tosinter together the grains of superhard material to form apolycrystalline superhard construction, the superhard grains exhibitinginter-granular bonding and defining a plurality of interstitial regionstherebetween, a non-superhard phase at least partially filling aplurality of the interstitial regions; wherein: the median of the meanfree path associated with the non-superhard phase divided by (Q3−Q1) forthe non-superhard phase is greater than or equal to 0.50, where Q1 isthe first quartile and Q3 is the third quartile of the mean free pathmeasurements associated with the non-superhard phase; and the median ofthe mean free path associated with the superhard grains divided by(Q3−Q1) for the superhard grains is less than 0.60, where Q1 is thefirst quartile and Q3 is the third quartile of the mean free pathmeasurements associated with the superhard grains; and the methodfurther comprising forming a non-planar surface topology in a firstsurface of the body of polycrystalline diamond material, the surfacetopology comprising one or more indentations in and/or projectionsextending from the first surface.
 27. The method of claim 26, wherein,the step of providing a mass of grains of superhard material comprisesproviding a mass of diamond grains having a first fraction having afirst average size and a second fraction having a second average size,the first fraction having an average grain size ranging from about 10 to60 microns, and the second fraction having an average grain size lessthan the size of the first fraction. 28-38. (canceled)
 39. The method ofclaim 26, wherein the step of forming the surface topology comprises:placing an aggregated mass of grains of superhard material into acanister; placing a ceramic layer formed of a ceramic material either indirect contact with the aggregated mass of grains of superhard material,or in indirect contact therewith wherein the ceramic layer is spacedfrom the grains by an interlayer of material, the ceramic layer having asurface with surface topology, the surface topology imprinting a patternin the aggregated mass of grains of superhard material complementary tothe surface topology, the ceramic material and the material of theinterlayer where present being such that they do not react chemicallywith the superhard material and/or a sinter catalyst material for thegrains of superhard material; the method further comprising: subjectingthe aggregated mass of grains of superhard material and ceramic layer toa pressure of greater than around 5.5 GPa in the presence of the sintercatalyst material for the grains of superhard material at a temperaturesufficiently high for the catalyst material to melt; sintering thegrains to form a body of polycrystalline superhard material having asurface topology complementary to the surface topology of the ceramiclayer; and removing the ceramic layer and said interlayer if presentfrom the body of polycrystalline material.
 40. A method according toclaim 39, wherein the step of placing the ceramic layer in contact withthe grains of superhard material comprises placing the ceramic materialin indirect contact therewith through the interlayer of material, theinterlayer comprising a coating on the ceramic layer.
 41. (canceled) 42.A method according to claim 39, wherein the step of placing the ceramicmaterial in contact with the grains comprises placing a ceramic materialformed of any one or more of the group of oxide ceramic materials thatare not reduced by carbo-thermal reaction in contact with the grains.43. A method according to claim 42, wherein the ceramic material isformed of any one or more of the group of oxide ceramic materialscomprising magnesia, calcia, zirconia, and/or alumina. 44-45. (canceled)46. A method according to claim 39, wherein step of forming the body ofpolycrystalline superhard material comprises forming a body having afree outer surface on removal of the ceramic layer therefrom in whichthe free outer surface is of the same quality as the bulk of the body ofpolycrystalline superhard material. 47-48. (canceled)
 49. A methodaccording to claim 39, wherein the step of placing the mass of superhardgrains into a canister comprises placing an aggregated mass of naturalor synthetic diamond grains into the canister. 50-51. (canceled)
 52. Amethod as claimed in claim 26, further comprising treating the body ofsuperhard polycrystalline material to remove catalyst material frominterstices between inter-bonded grains in the superhard material aftersintering. 53-56. (canceled)